The invention relates to a closed-cycle heat-engine of the kind known as a Stirling engine, adapted to convert heat supplied by a high-temperature source and rejected at a low-temperature sink, into mechanical energy by alternately compressing and expanding a gas and alternately heating and cooling it. The engine cycle may be reversed and the system is adaptable for refrigerating and heat pumping purposes. The known Stirling engines are being built to various lay-outs, all being characterized by that they comprise a working piston and a displacer piston reciprocatingly moving in two cylinders, th cylinders communicating through a regenerator which acts as a thermal storage device and alternately heats and cools the gas passing therethrough. All engines comprise a heater and a cooler which are separated by the said regenerator and are adapted to respectively heat and cool the gas while it is in contact with their heat-exchange surfaces.
In some Stirling engines the two cylinders are coaxial, i.e. they are in the form of a single, long cylinder, where a working and a displacing piston each are movable in approximation of the theoretical requirements of the Stirling cycle.
The ideal Stirling cycle comprises the following steps:
1. The gas is isothermally compressed in a "cold space". PA1 2. After compression the gas is transferred by the displacement piston to a "hot space", via the regenerator which heats it, at constant volume, to the temperature of the "hot space". PA1 3. The hot gas wholly contained in the "hot space" expands isothermally, by simultaneous movement of both the working and the displacementpistons. PA1 4. The gas is transferred to the "cold space" via the regenerator which cools it, at constant volume, to the temperature of the "cold space". PA1 1. Isothermal compression and expansion cannot be achieved during the rapid movement of the pistons. PA1 2. The regenerator volume is not zero, and the expansion and compression of the gas in this space results in a reduction in the specific output. PA1 3. Various drive mechanisms have been designed with the aim to move the pistons in a discontinuous motion allowing for the ideal cycle to prevail; however, since the mechanisms comprises gears, crankshafts and levers only sinusoidal approximation can be attained, whereby the ideal pressure/volume diagram is distorated. PA1 1. At the end of the expansion stroke the discs are in their "first" position, whereby the gas in the cylinder space is heated by the "heater" portion to a temperature close to the cylinder head temperature. PA1 2. The discs are brought, one by one, at great speed to their "second" position; while they traverse the hot gas they absorb its heat energy and cool it close to the piston temperature. PA1 3. During the compression stroke the gas remains at low temperature being cooled by the "cooler" portion, thus providing an isothermal compression. PA1 4. At the end of the compression stroke the discs are returned to their initial "first" position, whereby the gas is heated by the heater portion. PA1 5. The hot gas expands and drives the piston to the end of the expansion stroke, while the discs remain in their position keeping the gas at the high temperature, i.e. providing an isothermic expansion.
The actual cycle differs from the ideal cycle in many respects for the following reasons:
The result is that the engine efficiency is too low to make it competitive with other heat engines, also because the drive mechanism is complicated and, accordingly, expensive.